In the handling of suspensions or slurries, particularly sewage sludge or other flocculated or unflocculated slurries, it is often necessary to separate the liquid from the solid component of the suspension. For effective liquid-solid separation, devices such as belt filters and belt filter presses are employed.
A suspension is deposited on one end of a belt filter in an area of an apparatus commonly referred to as the "gravity drainage zone" or "gravity section". The belt can be supported by rollers or by perforated plates. Devices enhancing drainage or flow breaker devices are employed to enhance dewatering in the gravity section. Typical arrangements on a nominal two meter sized unit involve approximately ten rows of devices with about eight or nine devices per row. This ensures that all of the sludge in the gravity drainage zone contacts the devices for achieving maximum drainage of free water.
Without devices enchancing drainage or flow breakers, the solid component of the suspension may settle on the belt such that water can not pass by gravity through the belt. The stratified liquid and solid components then exit the zone without being effectively separated. Flow deflectors or similar devices are disclosed in U.S. Pat. No. 4,456,530 to Eustacchio et al, 3,984,329 to Wenzel et al, 4,602,998 to Goron and 4,609,467 to Morales.
By use of suitable flow breakers, the slurry blanket is split and the solid component is pushed aside exposing filter media cleared of slurry or solid material. With the devices enhancing drainage or flow breakers engaging the belt directly, openings generated within the solid component expose the belt to the free water and enable the free water to pass through the belt for collection below the filter belt. The solid component is then separately collected and discharged at an end of the filter belt opposite its inlet for downstream dewatering operations.
The systems disclosed in some of the above cited patents employ rigid upright elongated members of generally cylindrical or wedge shape as flow breakers for breaking or splitting the slurry and fail to provide adjustable pressure of the flow breakers against the belt. The conveyor flow breakers must be lifted, raised or pivoted out of the slurry for cleaning, interrupting operation. Additionally, the conventional flow breakers interrupt or deflect the slurry flow, rather than gently turn the slurry upside down. Due to the use of hard, non-flexing materials, the conventional flow breakers tend to abrade and to damage the belt.
As the slurry becomes denser and more viscous during the dewatering process on a gravity section, the upward force against the flow breakers generated by the solid component carried by the travelling belt increases in the direction of belt travel. The flow breakers will require increasing downward forces in the flow direction from one row to the next to maintain contact with the filter belt, and thus, assure maximum effectiveness.
With conventional flow breakers the downward force is predetermined by the weight of the flow breaker turning freely about its pivot axis or present by a locking mechanism. The downward force exerted by the flow breaker cannot be adjusted during operation. Furthermore, the downward force on individual flow breakers or rows of flow breakers is the same regardless of their position. Thus, conventional flow breakers are designed and perform at their best for only one operating condition and cannot be adjusted to the varying operating conditions existing on the gravity section of a belt filter or belt filter press. At locations spaced from this optimum operating point, the downward force will be either too high, resulting in accelerated wear of flow breaker and filter belt, or too low, resulting in poor performance of the flow breaker by being pushed out of the more dense slurry.